GB2620358A - A device for selectively retaining an object - Google Patents

Abstract

A device 100 for selectively retaining an object 200 for example releasably holding a fire door in the open position. The device 100 comprising a magnetic element 2 which can be demagnetised and means for moving the magnetic element between a first position and a second position. The magnet 2 can be demagnetised to release the door. The magnet element 2 is also moved to a second position further away from the door such that the magnet 2 may be retracted slightly, for example by a motor 8 and threaded shaft 16, within a housing to create a gap and lesson any residual magnetic attraction to improve release of the door. The magnet 2 may be a permanent electromagnet (electropermanent magnet). Preferably a controller may detect a wireless signal or a microphone detecting a fire-alarm to demagnetise and move the magnet which may only move a small distance ,e.g. between 2mm to 10mm. the device may be able to detect the position of the object/door. there may be a plurality of door retention devices controlled by a central server.

Description

A DEVICE FOR SELECTIVELY RETAINING AN OBJECT

Field of the Invention

The present invention relates to a device for selectively retaining an object. The invention also extends to corresponding methods and systems. The disclosure is particularly, but not exclusively, applicable to retaining a door, in particular a fire door.

Background

Fire doors are doors which are intended to be closed in the event of a fire within a building so as to form part of a safety fire break. As such, fire doors are usually biased such that they shut once open. However, for ease of access many fire doors are permanently held open with any suitable object that may be to hand. This is often, ironically, a fire extinguisher.

Door holders' for holding such fire doors open and releasing them in the event of an alarm condition are known. Certain 'door holders' are battery-powered and configured to be retro-fitted to be a door. Such 'door holders' may have limited functionality and may be constrained by the relatively high power that they consume during operation.

The present disclosure seeks to at least partially alleviate the problems outlined above.

Summary of the Invention

Aspects and embodiments of the present invention are set out in the appended claims. These and other aspects and embodiments of the invention are also described herein.

According to an aspect described herein, there is provided a device for selectively retaining an object, the device comprising: a magnetic element (for retaining the object); means for demagnetising the magnetic element (preferably selectively); and means for moving the magnetic element between a first position and a second position (preferably selectively).

The first and second position are preferably both within the device. Preferably, the magnetic element is configured (more preferably when magnetised) for engaging with the object whereby to retain the object.

Whether or not the object is retained of course depends on the strength of the magnet and the biasing of the door, as well as the distance of the first position from the second position. The strength of the magnet may therefore be selected based on the particular use-case for the device, in particular based on the strength of the biasing force associated with the object.

The device may further comprise a controller adapted to control the means for demagnetising and the means for moving.

Upon receipt of a predetermined signal, the controller is configured to control the means for demagnetising to demagnetise the magnetic element; and to control the means for moving the magnetic element to move the magnetic element to the second position.

Upon receipt of a predetermined signal, the controller may be configured to control the means for demagnetising to demagnetise the magnetic element; and simultaneously or immediately subsequently to control the means for moving the magnetic element to move the magnetic element to the second position.

The predetermined signal may be one of: a wireless data signal, a signal associated with a user-actuatable switch, and an (optionally audible) alarm, more preferably a fire alarm The device may further comprise means for detecting the predetermined signal, preferably wherein the means for detecting comprises a microphone; more preferably being in communication with the controller.

Upon receipt of a further predetermined signal, the controller may be configured to control the means for moving the magnetic element to move the magnetic element to the first position.

The means for demagnetising the magnetic element may be adapted to demagnetise the magnetic element by passing a current through the magnetic element.

The means for demagnetising the magnetic element may be adapted to demagnetise the magnetic element by passing a current through the magnetic element for a predetermined period of time, preferably wherein the predetermined period of time is selected such that the magnetic element moves at least part-way from the first position to the second position by the end of the predetermined period of time.

The predetermined period of time may be selected such that the magnetic element moves part-way from the first position to the second position by the end of the predetermined period of time. This can allow for a reduction in the time during which the magnet is demagnetised (which can be highly power-consuming) whilst allowing movement of the magnetic element near the first position (where the magnetic force to be overcome when moving the magnetic element can be greatest) when demagnetised, thereby reducing overall power usage of the device. The pad-way distance from the first position to the second position may be between 20% and 80% of the way/distance between the first position and second position, preferably between 40% and 60%, more preferably around 50%.

Alternatively, the predetermined period of time may be selected such that the magnetic element moves into the second position by the end of the predetermined period of time. This can allow for movement of the magnetic element only when demagnetised, thereby reducing power usage in moving the magnet (as a magnetic force need not be overcome).

The predetermined period of time may be less than 1 second, more preferably less than 500ms, yet more preferably between 100 and 200ms, still more preferably around 150ms.

The means for moving the magnetic element between a first position and a second position may be configured to move the magnetic element between the first and second positions along a straight line, preferably wherein said movement is respectively towards and away from an outer surface of the device.

The means for moving the magnetic element between a first position and a second position may comprise a motor configured to move the magnetic element between the first and second positions; preferably wherein the means for moving the magnetic element between a first position and a second position comprises a screw mechanism configured to receive rotational drive from the motor; more preferably wherein the magnetic element comprises a threaded passageway for receiving the screw mechanism thereby to allow the magnetic element to be driven along an axis of the screw mechanism.

The magnetic element may be moved in a passageway in a housing of the device, preferably wherein said passageway is open at an outer surface of the housing.

The device may comprise an outer surface, preferably facing the object, wherein the second position may be further away from the outer surface than the first position. The outer surface may be on the housing of the device, and preferably on the lid of the device.

The first and second positions may be spaced by between 2 and 10 mm, more preferably 2 and 5 mm, yet more preferably around 3 mm.

The magnetic element and the means for moving the magnetic element may be movable relative to an/the outer surface of the device. The magnetic element and the means for moving the magnetic element may be adapted to move away from the outer surface upon application of a force on the device, preferably on the outer surface of the device. Preferably, the magnetic element and the means for moving the magnetic element move together and/or in unison. The magnetic element and the means for moving the magnetic element may be freely movable relative to the outer surface. The device may comprise a resilient element for returning the magnetic element and the means for moving the magnetic element towards the outer surface upon application of a force on the device. The magnetic element and the means for moving the magnetic element are preferably movable relative to the outer surface by a distance of between 0.2mm and 2.0mm, more preferably between 0.4mm and 1.0mm, yet more preferably around 0.4mm.

The device may include one or more sensors adapted to detect a position of the magnetic element, preferably wherein the device is adapted to transmit data relating to the position of the magnetic element to a server or the controller.

A position of the magnetic element may be visible from the exterior of the device, preferably wherein the outer surface comprises an opening (preferably leading to a passageway) showing at least part of the magnetic element. Alternatively or in addition, the surface may be at least partly transparent and/or comprise a plurality of openings showing at least part of the magnetic element. The magnetic element is preferably visible via a lid of the device (more preferably, at least when the device is not retaining the object).

According to an aspect described herein, there is provided a device for selectively retaining an object, the device comprising: a magnetic element for retaining the object; and means for detecting the position of the object relative to the device by measuring a property of at least part of the magnetic element. As such, the same component may be used for retaining the object and for detecting the position of said object.

The controller may be adapted to detect the position of the object relative to the device; preferably wherein said detection includes detecting whether or not the object is engaged with the device and/or detecting whether or not the object is proximate the device.

The controller may be adapted to detect the position of the object relative to the device by controlling the means for demagnetising to demagnetise the magnetic element for a further predetermined period of time; and monitoring for a change in the inductance of at least part of the magnetic element. Controlling the means for demagnetising to demagnetise the magnetic element for a further predetermined period of time may include applying a (preferably DC) voltage pulse across the coil (of the means for demagnetising) for the further predetermined period of time. Monitoring for a change in the inductance of at least part of the magnetic element may include measuring the integral of current over time (in the coil) during the further predetermined period of time.

The further predetermined period of time may be less than the predetermined period of time; preferably wherein the further predetermined period of time is selected such that the object, if engaged with the device, remains engaged or is re-engaged by the device at the end of the further predetermined period of time; more preferably wherein the further predetermined period of time is less than 50ms, yet more preferably less than 20ms, still more preferably around 5ms.

For reduced power consumption, upon receipt of the predetermined signal (to release the object), if it is detected that the object is not proximate the device and/or not engaged with the device, the controller may be configured to control the means for moving the magnetic element to move the magnetic element to the second position (but preferably not to control the means for demagnetising to demagnetise the magnetic element, so as to save power).

The device may be battery-powered.

The device may further comprise means for detecting removal of a power source for the device. The power source may be attached to the device via an attachment formation. The means for detecting removal of a power source may comprise a switch adapted to actuate when said attachment formation (e.g. a captive screw) is detached. Upon detecting removal of a power source, the controller may be configured to: control the means for demagnetising to demagnetise the magnetic element; and (preferably simultaneously or immediately subsequently) to control the means for moving the magnetic element to move the magnetic element to the second position The device may be adapted for mounting to a surface, preferably a wall.

According to an aspect described herein, there is provided a device for selectively retaining an object, the device comprising: a magnetic element; means for detecting an occurrence of an alarm, preferably a fire alarm; means for demagnetising the magnetic element, preferably upon detection of an alarm.

According to an aspect described herein, there is provided a method of releasing an object retained by a magnetic element on a retaining device, the device preferably being the device of any preceding claim; the method comprising: demagnetising the magnetic element; and moving the magnetic element from a first position to a second position, preferably wherein, in the first position, the magnetic element is capable of engaging with the object whereby to retain the object; and, in the second position, the magnetic element is not capable of engaging with the object whereby to retain the object.

According to an aspect described herein, there is provided a system comprising: the device described herein; and an object to be retained by the device. The object may include a magnetic element; preferably wherein said magnetic element is adapted for engagement with a/the magnetic element of the device.

The system may further comprise means for mechanically biasing the object away from the device, preferably such that the object moves away from the device when the device selectively disengages from the object. Preferably, the object is a door. The object may be adapted to cover an opening, preferably wherein the object is a door. Preferably, the means for mechanically biasing is adapted to close the door when not retained by the device.

According to an aspect described herein, there is provided a system comprising: a plurality of devices as described herein; and a server adapted to transmit a signal to each device to release the object retained by each device from being retained by the device.

According to an aspect described herein, there is provided a device for selectively retaining an object, the device comprising: a magnetic element; means for moving the magnetic element between a first position and a second position; and an outer surface; wherein the magnetic element and the means for moving the magnetic element are movable relative to the outer surface.

The magnetic element and the means for moving the magnetic element may be adapted to move away from the outer surface upon application of a force on the outer surface.

According to an aspect described herein, there is provided a device for selectively retaining an object, the device comprising: an attractive element; means for (temporarily) deactivating or masking the attractive element (preferably selectively); and means for moving the magnetic element between a first position and a second position (preferably selectively).

According to an aspect described herein, there is provided a device for selectively retaining an object, the device comprising: a magnetic element; means for detecting occurrence of an alarm, preferably a fire alarm; and means for demagnetising the magnetic element upon detection of the occurrence of the alarm.

According to an aspect described herein, there is provided a device for selectively retaining an object, the device comprising: a magnetic element; a module for demagnetising the magnetic element (preferably selectively); and a module for moving the magnetic element between a first position and a second position (preferably selectively).

According to an aspect described herein, there is provided a device for selectively retaining an object, the device comprising: a magnetic element; means for moving the magnetic element between a first position and a second position (preferably wherein at least one of said positions is within the device); wherein a position of the magnetic element can be seen by a user, preferably wherein an outer surface comprises an opening showing at least part of the magnetic element.

According to an aspect described herein, there is provided a device for retaining an object, the device comprising: a magnetic element; an outer surface; and means for moving the magnetic element between: a first position closer to the outer surface, and a second position further from the outer surface; preferably wherein said movement is linear/in a straight line.

According to an aspect described herein, there is provided a device for retaining an object, the device comprising: a base; a magnetic element; and means for pivoting the magnetic element about the base.

The means for pivoting may comprise a ball joint.

The device may be adapted for engaging with the device for selectively retaining an object as described herein (preferably via the magnetic elements thereof).

The device may be a door keeper device.

Any apparatus feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure, such as a suitably programmed processor and associated memory, for example.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination. It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.

The invention extends to methods, system and apparatus substantially as herein described and/or as illustrated with reference to the accompanying figures.

As used herein, the term "magnetic element" preferably connotes a component which exhibits properties of magnetism, preferably wherein said component is attracted to a further component such that the component and further component may engage, i.e. may be held together, more preferably wherein the magnetic element is a permanent electromagnet. The term "magnetic element" as used herein is synonymous with the term "electromagnetic element".

As used herein, the term "demagnetise" (and similar terms such as "demagnetised"), as used in relation to demagnetising a magnetic element, preferably connotes causing a relative reduction in the magnetic field strength of the magnetic element; more preferably a sufficiently large reduction in the magnetic field strength of the magnetic element such that the magnetic element and a retained object disengage (i.e. move apart), for example by a mechanical bias and/or weight of an object being greater than the (reduced) attractive force between the magnetic element and the object. As used herein, the term "engage" (and similar terms such as "engagement"), as used in relation to engagement between a magnetic element, preferably connotes the presence of a sufficiently strong attractive force between the magnetic element and object to allow the first and second objects to be retained relative to one another, for example, for example by a mechanical bias and/or weight of an object being less than the attractive force between the magnetic element and the object.

It should be noted that the term "comprising" as used in this document means "consisting at least in part of". So, when interpreting statements in this document that include the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner. As used herein, "(s)" following a noun means the plural and/or singular forms of the noun.

Brief Description of the Drawings

One or more aspects will now be described, by way of example only and with reference to the accompanying drawings having like-reference numerals, in which: Figure 1 is a system including a retaining device; Figure 2a is a perspective view from above of the device; Figure 2b is a front view of the device; Figure 3 is an exploded view of the device; Figure 4 is a cutaway view of the device along the line A-A shown in Figure 2b; Figure 5 is a cutaway perspective view of the device; Figure 6 is a perspective view of a reflector plate; Figure 7 is a schematic side view of an engaging element; Figure 8a is an exploded view of the engaging element; Figure 8b is a bottom view of the engaging element; Figure 8c is a side view of the engaging element; Figure 8d is a cutaway side view of the engaging element along the line A-A shown in Figure Sc; Figure 9 is a method of retaining a door in the system; Figure 10 is a cutaway perspective view of the device and the engaging element; Figure 11 is a top view of the device and the engaging element; and Figure 12 is a cutaway side view of the device and the engaging element along the line E-E shown in Figure 11.

Detailed Description of Preferred Examples

Figure 1 shows an example system 1000 incorporating a retaining device 100. The device 100 is mounted on a wall 500 (for example of a corridor) adjacent a door 400 that covers an opening 500. As is conventional, the door 400 (shown with the door leaf facing out of the figure) moves on hinges 420 and includes a handle 450. The door 400 also includes a biasing means 300 in the form of a door closer, which is attached to the door 400 and the lintel of the opening so as to urge (i.e. bias mechanically) the door 400, when opened, into a closed position. The use of such a door closer is typical for fire doors and other doors which should usually be in a closed position apart from when a user passes through them. On the side of the door 400 that approaches the wall 500 (and so device 100) when the door 400 is open, there is provided an engaging element 200. The engaging element 200 and device 100 are arranged such that, when the door 400 is fully opened, the engaging element 200 and retaining device 100 are in contact or otherwise are located very close to each other (it will be appreciate that the exact locations and sizes of the engaging element 200 and retaining device 100 shown in the figure are merely exemplary). The device 100 is configured to engage with the engaging element 200 selectively so as to retain selectively the engaging element 200, thereby to retain the door 400 in an open position (against the biasing force provided by the biasing means 300). When the device 100 and the engaging element 200 disengage, the door 400 closes under the bias of the biasing means 300.

Retaining device Figures 2a to 5 show an example retaining device 100. The device 100 includes a magnetic element 2, means for demagnetising the magnetic element, and means for moving the magnetic element between a first position and a second position. The device 100 uses the magnetic element 2 to engage with the engaging element 200 magnetically so as to retain the door 400. The retaining of the door 400 is made selective by providing means for demagnetising the magnetic element to release the engaging element 200 and thereby the door 400 (where the biasing of the door means that the engaging element 200 moves away from the device 100). As will be explained, the movement of the magnetic element 2 may provide further security against the door 400 being retained open (for example by accident) when it should be biased into a closed position.

The device 100 comprises a housing comprising: a lid 1, a base 5, and a wall portion 4 which serves to connect the lid 1 and base 5. The housing houses, among other components, the magnetic element (or 'magnet') 2, an electric motor 8, a coupler 16 between the motor 8 and the magnet 2, and a printed circuit board (PCBA) 6 which acts as a controller for the device 100. The PCBA 6 provides the aforementioned means for demagnetising the magnetic element 2. The coupler 16 and motor 8 provide the aforementioned means for moving the magnetic element, where -as will be described -the coupler 16 forms a screw mechanism for moving the magnet 2.

The housing of the device 100 is shaped to facilitate attachment to a mounting surface (e.g. to the wall 500), and to facilitate engagement with the engaging element 200. An outer wall of the base 5 is flat. The base 5 (and so the device 100 as a whole) is generally square-shaped. The magnet 2 is provided centrally in the lid 1 of the device 100, in an opening 19. This may allow a user to inspect the magnet 2 from the exterior and in particular to determine its position within the device 100 The magnet 2 is generally cylindrical in shape, where a flat top surface of the magnet 2 may be aligned with the lid 1. The magnet 2 is a permanent electromagnet (also referred to as an electropermanent magnet) -e.g. a 8V, 0.7A alnico or ferrite permanent electromagnet -and is rated to hold EN 1154 standard door closers in at least sizes 1-5, preferably sizes 1-7, or a 25 kg load. As is conventional for permanent electromagnetics, the magnet 2 includes a pole piece magnetised by a permanent magnet, and a coil of wire (not shown), wound around the pole piece, which is used to control the magnetisation of the magnet 2.

The lid 1 is sloped or curved from the position of the magnet 2 to the wall portion 4, such that the magnet 2 is at the outermost point of the device 100 from the base 5. The magnet 2 is surrounded by a flat, generally square-shaped top portion 29 of the lid 1. Four slots 27 are provided in the lid 1, extending from within the square-shaped portion 29 to the edges of the lid 1. In the slots 27, apertures 22 are provided towards the edge of the lid. The lid 1 (and therefore the device 100) can be attached to the mounting surface 500 via screws (e.g. self-tapping screws 13 arranged through the apertures 22 in the lid 1. Along with providing room for the apertures 22, the slots 27 may serve to draw a user's attention to the central magnet 2 in the lid 1. The lid 1 and base 5 are attached to one another via screws (not shown) -e.g. via self-tapping screws. The base 5 includes an upwardly standing perimeter wall 23 which fits within the separate wall portion 4. The base 5 and wall portion 4 are attached to one another via respective engaging formations 24, 25 in the wall portion 4 and base 5 respectively.

The PCBA 6 is attached to the base 5 via screws 12 arranged through apertures 22 in the PCBA 6. The motor 8 is attached to the base 5 and is arranged such that a drive shaft 28 of the motor 8 extends through a gap in the PCBA 6. The motor shaft 28 is arranged centrally in the device 100. The motor 8 is used to move the position of the magnet 2 within the device 100. A damper 7 is arranged between the motor 8 and the base 5. The damper 7 can dampen vibration from the motor 8 and reduce transmission of said vibration to the base 5. Further, the damper 7 can dampen the impact on the drive shaft 28 and motor 8 caused by the engaging element 200 on the door 400 coming in contact with the device 100 when the door 400 is fully opened. The damper 7 may, for example, comprise a rubber damper strip. The damper 7 may further comprise an elastic foam layer (not shown) that compresses upon impact by the motor 8 (caused by the engaging element 200 on the door 400 coming in contact with the device 100), which may provide further dampening and shock absorption (e.g. such that the device 100 may be rated to hold EN 1154 standard door closers in size 7). The foam layer is resilient such that it expands to its original size subsequent to an impact caused by the caused by the engaging element 200 on the door 400 coming in contact with the device 100, and may thereby assist in repositioning the motor 8 following such an impact (as is described in further detail below). As shown in Figure 3, the motor 8 is provided on the damper 7, and the damper 7 is provided on the base 5.

The device 100 is battery powered. This can provide for easy retro-fitting of the device 100 as no wiring is needed to install the device 100 on the wall 500. As such, the device 100 comprises a battery carrier 14 which is removably attached to the housing of the device 100. The wall portion 4 (and optionally the perimeter wall 23 of the base 5) comprises an aperture 33 for receiving the battery carrier 14. The outer surface of the wall portion 4 / perimeter wall 23 is shaped to match the shape of the aperture 33 to provide a continuous outer wall surface for the device 100 when assembled. The battery carrier 14 can be attached to the wall portion 4 via a captive screw 9 and a corresponding retaining clip 11, and/or, for example, via a bayonet or snap fit mechanism. This may allow for the battery carrier 14 to be removed from the side of the device 100 so that the batteries can be replaced while the device 100 remains in situ. The battery carrier 14 comprises a battery clip 15 which can house a three-cell battery pack 10 (e.g. an AA parallel lithium iron disulfide battery pack). The battery pack 10 powers the PCBA 6 as well as the motor 8. The battery pack 10 disengages from terminal contact pins (not shown) in the device 100 when the captive screw 9 is unscrewed and the battery carrier 14 is removed from the side of the device 100. The device 100 further comprises a tamper switch (not shown) that is activated by the captive screw 9 in normal use. The captive screw 9 deactivates the tamper switch upon removal/unscrewing of the captive screw 9. When the tamper switch is deactivated, the PCBA 6 arranges the device 100 in a fail-safe condition -e.g. to release the door 400 and/or cause the device to be incapable of engaging the door (via steps 604606 of method 600 described in further detail below) -before the battery pack 10 is completely removed. This can improve the safety of operation of the device 100. The lid 1 comprises a downwardly/inwardly depending flange 18 in which the magnet 2 sits (best visible in Figure 4). The flange 18 is shaped to match the shape of the magnet 2 so that the magnet 2 can move along it, such that the flange 18 may be described as a passageway for the magnet 2. As described in further detail below, the magnet 2 is movable along the passageway 18 between a protruded position (shown, e.g. in Figure 4) in which the upper surface of the magnet 2 is generally parallel to the flat lid portion 29 (and preferably where a lower surface of the magnet 2 is generally parallel to a bottom edge 30 of the flange 18), and a retracted position in which the magnet 2 is positioned further away from lid portion 29 (such that the upper surface of the magnet 2 is within the passageway 18 and the lower surface is below the end of the passageway 18). The magnet 2 is attached at its lower end to a plate 3 which is used to delimit movement of the magnet 2. The protruded position of the magnet 2 is reached when the plate 3 comes in contact with a lower edge 30 of the flange 18. The plate 3 extends beyond the side of the magnet 2 and the edges of the flange 18 so that the guiding portion 18 retains the plate 3 and the travel of the magnet 2 is limited. The magnet 2 includes an aperture in its lower surface, extending through a major part of the magnet 2 to form a passageway 34. The passageway 34 is threaded. A threaded coupler 16 engages with the threaded passageway 34, where the coupler 16 is driven by the shaft 28 of the motor 8. As such, when the motor 8 is driven in one direction, the rotation of the coupler 16 in the passageway 34 causes (as a result of the constrained rotation of the magnet 2) the magnet 2 to move axially away from the motor 8 (towards the protruded position). When the motor 8 is driven in the other direction, the magnet 2 moves towards the motor 8 (towards the retracted position). The lowermost/retracted position of the magnet 2 is reached when the tip of the coupler 16 comes in contact with or closely approaches the end of the passageway 34 in the magnet 2. As mentioned, the travel of the magnet 2 is limited such that the magnet 2 cannot be driven such that the coupler 16 disengages with the aperture 34 The protruded and retracted positions are arranged such that in the protruded position, the magnet 2 can engage with the further magnetic element 202 on the object (e.g. door 400) and retain the door 400; and, in the retracted position, the magnet 2 is retracted far enough away from the further magnetic element 202 that the two do not engage (or are not attracted sufficiently strongly to retain the door 400) when the engaging element 200 is arranged adjacent the door 400.

In a preferred example of the device 100, shown in Figures 10 to 12, the motor 8 is arranged in the device 100 such that it can freely move towards and away from the lid 1 by a small distance (e.g. between 0.2mm and 1.0mm, preferably around 0.4mm). The motor 8 can move between: an extended position in which the motor's top surface 52 is in contact with a bottom surface 50 of the PCBA 6 and a gap is provided between a bottom surface 54 of the motor 8 and a top surface 56 of the damper 7, and a receded position (shown in Figure 12) in which the motor bottom surface 54 is in contact with the damper top surface 56 and a gap is provided between the motor top surface 52 and PCBA bottom surface 50. The coupler 16 and magnet 2 are also arranged to move in unison with the motor 8 between its extended and receded positions. In the receded position of the motor 8, coupler 16 and magnet 2 (shown in Figure 12), the magnet 2 (when in its protruded position) is receded by the same small distance as the motor 8 from the top portion 29 of the lid 1 (where this small separation between the magnet 2 and top portion 29 of the lid 1 is not shown in Figure 12). In the extended position of the motor 8, coupler 16 and magnet 2 (when in its protruded position), the magnet 2 is generally aligned with the top portion 29 of the lid 1 and the motor top surface 52 is in contact with the PCBA bottom surface 50. In either of the receded and extended positions of the motor, the magnet 2 (in its protruded position) can come in contact with and engage with the further magnetic element 202 of the engaging element 200 because the magnetic attraction force between them is strong enough to move the magnet 2, coupler 16, and motor 8 from their receded positions to their extended positions. Thus, the magnet 2 is able to engage with the further magnetic element 202 when the motor is in either position.

Providing a small amount of 'play' in the position of the magnet 2, coupler 16, and motor 8 in this way can improve the resilience of the device 100 to external forces being applied on the magnet 2 exposed via opening 19. If the magnet 2 (in its protruded position) were not able to move into the device slightly by way of the motor 8 moving to a receded position, the application of a large force on the device 100 (e.g., as a result on a person pushing on the door 400 when retained by device 100) may result in a large frictional force between the threaded passageway 34 of the magnet 2 and the threaded coupler 16, which may cause the binding of the two threads leading to stalling. By allowing movement of the magnet 2 (alongside the motor Sand coupler 16), this problem can be mitigated, because, upon application of said large force on the device 100, the magnet 2 (and motor 8, and coupler 16) move to their receded positions and the lid 1 absorbs the majority of the force, thereby preventing binding of the magnet 2 and coupler 16 threads. The damper 7 can further be used to dampen the impact on the motor 8 when it is moved to its receded position upon a force being exerted on the device 100. The damper 7 (via expansion of its elastic foam layer) can further act to return the motor 8 (and thereby the magnet 2) towards its extended position after the impact.

The motor 8 may be a right angled, geared DC motor. The motor 8 comprises a drive outlet 28 for driving the coupler 16. The motor 8 provides a rotational force to the drive outlet 28, which, in turn, rotates the coupler 16. The coupler 16 is a threaded screw 16 that is rotationally fixed to the drive outlet 28 (i.e. the coupler 16 rotates in unison with the drive outlet 28).

The drive of the motor 8 and movement of the magnet 2 is controlled by the PCBA 6. The PCBA 6 receives input from one or more sensors that detect movement and/or the position of the magnet 2, processes this input, and outputs a control signal for the motor 8. In the present example, the one or more sensors comprise a position sensor 36 and a movement/rotation sensor 40. The position sensor 36 detects when the magnet 2 is at one of its extreme positions (i.e. at one of the protruded or retracted positions). Specifically, the position sensor 36 detects when the magnet 2 is in the retracted position. The position sensor 36 comprises an infrared photosensor and is actuated by a (optionally metallic) member 38 protruding towards the base 5 (and position sensor 36 mounted on the PCBA 6) from the plate 3. In addition to detecting when the magnet 2 is at one of its extreme positions, the position sensor 36 is also used to calibrate the rotation sensor 40. The rotation sensor 40 detects rotation of the coupler 16 / drive shaft 28, based on which the PCBA 6 can compute the position of the magnet 2. The rotation sensor 40 comprises an optical reflective sensor 40. One or more apertures (not shown) are provided in the base 32 of the coupler 16 to assist in detection of rotation of the coupler 16 (by providing a variable reflective pattern as the coupler 16 rotates). Alternatively, or in addition to apertures in the base 32, a reflector plate 42 (shown in Figures 5 and 6) may extend around the coupler 16 and the reflective sensor 40 may detect rotation of the reflector plate 42. As shown in Figure 5, the reflector plate 42 is attached around the coupler 16, just below the base 32. As shown in Figure 6, the reflector plate 42 comprises a central aperture 44 shaped to receive coupler 16, and alternating arms 46 and indents 48. When the coupler 16 and reflector plate 42 rotate, the alternating arms 46 and indents 46 provide a variable reflective pattern that can be detected by the reflective sensor 40.

The PCBA 6 provides means for demagnetising of the magnet 2. In the present example, the magnet 2 is a permanent electromagnet so, in its 'rest' state the magnet 2 is magnetised. The magnet 2 is arranged such that its magnetic axis is aligned with the axis of rotation of the drive outlet 28 and coupler 16, so that the magnet 2 is moved between the protruded and retracted positions along its magnetic axis. When current is supplied (i.e. voltage is applied) to the coil of the magnet 2 by the PCBA 6, it induces an opposite magnetic field in part of the permanent electromagnet 2 to that produced by another part of the permanent electromagnet 2, and so demagnetises the permanent electromagnet 2. Once current supply to the coil is stopped, the opposite magnet field is no longer induced, and so the magnet 2 (as a whole) is re-magnetised. This cycle of magnetisation, demagnetisation, and re-magnetisation can be repeated multiple times. Using a permanent electromagnet (as opposed to, e.g., an electromagnetic circuit that requires a constant power supply to be in the magnetised state), and only temporarily demagnetising the magnet 2, can reduce power usage of the device 100, and so can allow a battery-powered (as opposed to mains-powered) device 100 to be used without requiring the need for very regular changes of the battery.

The device 100 also includes a manual release switch. The release switch comprises a push button switch 20 provided in an aperture 26 in a side of the device 100 (specifically, in an aperture 26 formed by the shapes of the wall portion 4 and base 5. When a user presses the button of the switch 20, a corresponding signal is sent to the PCBA 6, which in turn sends a control signal to the motor 8 and/or coil to release the retained object (e.g. to demagnetise the magnet 2 and move the magnet 2 to the retracted position).

The PCBA 6 also provides means for detecting an alarm On particular, a fire alarm); and a means for communicating with an external server. In this regard, the PCBA 6 includes one or more sensors for detecting an alarm. The PCBA 6 also includes a transmitter and receiver (not shown) for communicating with a server.

The one or more sensors for detecting an alarm comprise a radio sensor and an acoustic sensor/microphone that is activated by sound waves from the alarm. For example, the radio sensor may comprise a radio receiver that receives a radio signal from a transmitter linked to a fire alarm panel, the radio sensor being activated when the transmitter indicates a fire state. The use of two different sensors may provide a degree of redundancy. Once an alarm is detected, the sensors provide an input to the PCBA 6 which actuates the motor 8 and the coil to release the door 400.

The PCBA 6 may, for example, use the transmitter and receiver to receive control inputs from the external server to control the device 100 to release the door 400 or to allow the door 400 to be retained, and/or may transmit the current state of the magnet 2 (e.g. magnetised or demagnetised, in retracted position or in protruded position) and/or data relating to position of the door 400 relative to the device 100 (and thereby whether the device 100 is currently retaining the door 400) to the external server.

Engaging element Figure 7 shows a schematic diagram of an engaging element 200. The engaging element 200 comprises: a base 206, an arm 204 extending from the base 206 at one end, and a further magnetic element 202 (mounted on the end of the arm 204 away from the base 206) that engages with the magnet 2 on the device 100. The further magnetic element 202 is made from a ferromagnetic material, such as iron, nickel, cobalt, or steel. Alternatively, the further magnetic element 202 may be a permanent electromagnet arranged to attract the magnet 2 (i.e. with the North/South pole of the electromagnet facing the South/North pole of the magnet 2).

The base 206 has a flat surface 208 which can be mounted to a surface of the door 400, e.g. using an adhesive or screws. In the present example, the base 206 has a square top-view shape (not shown), and the arm 204 and the further magnetic element are cylindrical.

Figures 8a to 8d show an example engaging element 200. The engaging element comprises a magnet 202, base 206, and arm. The arm comprises a magnet housing 212, a ball 210, and a fixing element 208 for attaching the magnet 202, magnet housing 212, base 206 and ball 210 to one another. The magnet 202 is a permanent electromagnet, arranged to attract the magnet 2 of the device 100. The base 206 comprises apertures 214 shaped to receive fixings (e.g. screws) for attaching the base 206 to a surface (e.g. the door 400). The magnet housing 212 comprises an opening 226 shaped to receive the magnet 202. As shown in Figures 8c and 8d, the magnet 202 slightly protrudes beyond the magnet housing 212 to allow direct contact with the magnet 2 of the device 100.

The top of the base 206 includes an opening 232 for the ball 210. The ball 210 protrudes through the opening 232 and can rotate in the opening 232. The ball 210 is fixedly attached to the magnet housing 212 via the fixing element 208. The ball 210 thus forms a ball joint for pivoting the magnet housing 212 about the base 206, which can allow easier alignment of the magnet 202 with the magnet 2 of the device 100 (e.g. if the door 400 is slightly slanted).

As shown in Figure 8a, the side walls 216 and 218 of the base 206 and magnet housing 212 respectively are curved so as to allow for greater freedom of movement of the base 206 and magnet housing 212 relative to one another about the ball joint formed by the ball 210. The ball 210 further comprises a protrusion 228 shaped to fit an indent 230 in the magnet housing 212 which can provide for a more secure fit between the ball 210 and magnet housing 212.

The fixing element 208 is a screw -e.g. a flat head socket cap screw. Each of the ball 210, magnet housing 212, and magnet 202 comprise an aperture for receiving the fixing element 208 -see apertures 220, 222, and 224 respectively in Figure 8d. The aperture 224 in the magnet 202 is threaded (as shown in Figure 12). Accordingly, when the fixing element 208 is inserted through apertures 220, 222, and 224, the thread of the fixing element 208 can engage the threaded aperture 224 in the magnet 2, thereby attaching the magnet 202, magnet housing 212, base 206 and ball 210 to one another.

As shown in Figure 8b, the engaging element 200, base 206, and magnet 202 have a generally square top-view shape. As also shown in Figure 8b, the base 206 is generally hollow, which can reduce the mass of the engaging element 200.

Figures 10 to 12 show the device 100 of Figures 2a to 5, and the engaging element 200 of Figures 8a to 8d when engaged to one another. As shown in Figures 10 to 12, when the device 100 and engaging element 200 are engaged, the magnet 2 and further magnetic element 202 are in contact with one another and are generally parallel.

Retaining a door Figure 9 shows an example method 600 of retaining a door 400 in the system 1000 of Figure 1 using the device 100. At the start of method 600, the magnet 2 of the device 100 is engaged with the further magnetic element 202 on the engaging element 200, and the door 400 is retained in its open position by the device 100.

First, at step 602, the device 100 receives a signal to release the door 400. The signal may be, for example, a conventional audible fire alarm which is detected via the acoustic sensor/microphone of the PCBA 6. Alternatively, the signal may be received via the manual release switch 20, which is detected by the PCBA 6.

Next, at step 604, in response to the receipt of the signal the PCBA 6 actuates (i.e. actuates current supply to) the coil to demagnetise the magnet 2. When the magnet 2 is demagnetised, the magnetic (attractive) force between the magnet 2 and the further magnetic element 202 on the engaging element 200 is insufficiently large to compensate for the force exerted by biasing means 300 on the door 400 away from the device 100, so the door 400 is released and moves away from the device 100 and wall 500 to its closed position. The magnet 2 is demagnetised only temporarily to conserve power (since demagnetising the magnet 2 requires a current to be supplied to the coil and can consume a significant amount of power). The magnet 2 is demagnetised for up to 1 second, more preferably for up to 500ms, yet more preferably for between 100 and 300ms. The applied current is generally around 1A.

Next, at step 606, the PCBA 6 actuates the motor 8 and coupler 16 to move the magnet 2 towards the base 5 (i.e. away from the engaging element 200) to its retracted position. Moving the magnet 2 to its retracted position ensures that the magnet 2, when remagnetised, cannot be engaged with the engaging element 200. This means that the door 400 returns to a closed position even when fully opened by a user, e.g. such that the device 100 and engaging element 200 contact each other Steps 604 and 606 are initiated immediately subsequently (or even substantially simultaneously), such that the magnet 2 is, at least partly, moved to its retracted position during the time that it is demagnetised. As such, the torque that the motor 8 needs to provide may be reduced as, while the magnet 2 is demagnetised, the motor 8 does not need to provide sufficient torque to overcome the magnetic attractive force of the magnet 2 and engaging element 200, which may reduce the power consumption of the motor 8. In a preferred example, steps 604 and 606 are initiated substantially simultaneously.

The magnetic attractive force of the magnet 2 and the engaging element 200 is strongest when the magnet 2 is in its protruded position and said attractive force exponentially decreases as magnet 2 is moved to its retracted position. Thus, the power consumption of the motor 8 can be particularly reduced by initiating steps 604 and 606 immediately subsequently (or substantially simultaneously) thereby removing the need for the torque of the motor 8 to overcome said attractive force when it is strongest.

However, steps 604 and 606 need not be of the same duration -in particular, the magnet 2 can be demagnetised 604 for a shorter period of time than the time taken to retract 606 the magnet 2 to its retracted position. Demagnetising the magnet 2 can require a relatively large power input (e.g. at around 8V voltage and 1A current). Once the magnet 2 is sufficiently far from its protruded position, demagnetising the magnet 2 can consume more power than would have been saved by the motor 8 by demagnetising the magnet 2. Thus, demagnetising the magnet 2 for a shorter period of time than the time taken to move the magnet 2 to its retracted position can allow balancing the power consumption of the motor 8 and coil, and can provide for a reduction in the overall power consumption of the device 100. For example, the magnet 2 may be demagnetised for around 200ms, whereas it may take around 350ms for the magnet 2 to be moved to its retracted position.

Once the magnet 2 is moved 606 to its retracted position (or at least part-way to the retracted position, as described above), the magnet 2 is re-magnetised, at step 608. Retracting the magnet 2 may ensure that the device 100 cannot retain the door 400, so the magnet 2 can be safely re-magnetised (i.e. current supply to the coil stopped) to conserve power. As mentioned, the re-magnetisation of the magnet 2 is based on a predetermined time period elapsing, rather than e.g. the position of the magnet 2 being detected, but in an alternative the position sensor may be used to sense a position of the magnet 2 and the magnet 2 may only be re-magnetised when the magnet 2 is detected as being in the retracted position Next, at step 610, the device 100 retains the magnet 2 in its retracted position, whilst awaiting a further signal. It will be appreciated that step 610 corresponds to a stable position that consumes minimal power. The only power consumed in this state is the power used in operating the PCBA 6 and sensors -no power is consumed in moving or demagnetising the magnet 2.

At step 612, the device 100 receives a further signal to allow the door 400 to be retained (i.e. to 'arm' the device 100). This further signal may be received via a communication received via the communication components on the PCBA 6 or via the manual release switch 20.

Finally, in response to the signal received at step 612, at step 614, the device 100 moves the magnet 2 to its protruded position using the motor 8 and the coupler 16. After step 614, the magnet 2 and device 100 are therefore ready for engagement with the engaging element 200 to retain the door 400.

The above-described method 600 may provide an efficient and safe method of selectively retaining a door 400. In particular, method 600 can reduce power usage of the device 100 and so allow it to be battery-powered (and for batteries to last longer).

In more detail, the device 100 and method 100 provides two stable states: 1) a state in which the device 100 can be engaged, in which the magnet 2 is magnetised and in its protruded position (i.e. the state after step 614), and 2) a state in which the device 100 cannot be engaged, in which the magnet 2 is magnetised and in its retracted position (i.e. the state after step 608). Neither of these two stable states consume substantial power, i.e. they do not consume any more power than is necessary to operate the PCBA 6 and sensors running. The magnet 2 is a permanent electromagnet so no power is required to maintain the magnet 2 in its magnetised state. Similarly, once the magnet 2 is moved to its retracted or protruded position, the device 100 can retain the magnet 2 there without consuming any power -the magnet 2 is retained by the coupler 16 so once the motor 8 stops rotating the coupler 16, no further power input is required to retain the magnet 2 in its position.

Further, method 600 may provide for efficient switching between the two states. In particular, to switch from the state in which the device 100 can be engaged to the state in which the device 100 cannot be engaged, the magnet 2 is demagnetised for only a brief period of time and retracted whilst demagnetised (after which it is re-magnetised). This can allow minimising the amount of power required to demagnetise the magnet 2 (since it is demagnetised only briefly) and to retract the magnet 2 (since it is retracted whilst demagnetised and so the magnetic attractive force between the magnet 2 and the further magnetic element 202 need not be compensated for), which can reduce overall power usage. Further, once the magnet 2 is demagnetised, to move the magnet 2 between its protruded and retracted positions, the magnet 2 is moved axially (i.e. towards/away the engaging element 200) by a short distance (preferably around 3mm) which can reduce power usage as compared to moving the magnet 2 in another direction. If the magnet 2 was not demagnetised, moving it between the first and second positions (e.g. rotating the magnet 2 out of magnetic engagement) would consume significantly more power.

Detecting the position of the door The PCBA 6 and the coil of the permanent electromagnet 2 further provide means for detecting the position of the door 400 relative to the device 100 (and thereby whether the device 100 is retaining the door 400) at a given time. When the device 100 is retaining the door 400, the further magnetic element 202 is located proximate to (or even in contact with) the permanent electromagnet 2 and therefore proximate to its pole piece, around which the permanent electromagnet's coil is wound, and which is magnetised by a permanent magnet. The proximity of the further magnetic element 202 to the pole piece concentrates the magnetic flux at the junction between the pole piece and further magnetic element 200. This concentration of magnetic flux increases the inductance of the coil (for example, by a factor of around 10 as compared to the inductance of the coil when the device 100 is not retaining the door 400 (e.g. when the door 400 is closed as opposed to being retained open) and the further magnetic element 202 is located further from the device 100). The PCBA 6 is adapted to detect this variation in inductance of the coil by applying a short DC voltage pulse of predetermined time period to the coil, monitoring the resulting increase in current, and by measuring the integral of current over time during the predetermined time period. The integral of the current over time during the predetermined time period increases approximately proportionally with increasing inductance, and so can be measured to detect a change in the inductance, and thereby to detect the presence or otherwise of the door 400 proximate to the device 100. It will be appreciated that the variation in inductance between the two states (i.e. further magnetic element 202 being proximate or distant from the coil and device 100) can be calibrated (e.g. at the time of production of the device 100 or periodically over the lifetime of the device 100) and a threshold set for the integral of the current over time, based on which threshold a determination can be made by the PCBA 6 whether the engaging element 200 (and door 400) is proximate the device 100 or not and a corresponding (binary) output signal produced.

The magnet 2 therefore effectively provides two functions -holding and releasing the engaging element 200 (and door), and detecting whether or not the engaging element 200 (and door) are held by the device (that is, detecting the position of the engaging element 200 (and door 400) relative to the device). Using a single component for both of these functions can simplify manufacture of the device 100. To ensure that the engaging element 200 is not accidentally released when its presence is detected, the voltage pulse applied to the coil (based on which inductance of the coil is measured) is selected to be sufficiently short so that the engaging element 200 is not released or is released but moves only a very small distance (on the swing door, which is biased towards a closed position) such that the engaging element is immediately 'recaptured' by the magnet at the end of the pulse, but sufficiently long to be able to detect a change in inductance of the coil. For example, the voltage pulse may be applied for around 5 milliseconds.

Detecting the position of the door 400 / engaging element 200 relative to the device 100 may provide useful data for a user, e.g. a controller of a security system, wishing to know whether doors 400 in a building are held or not. Detecting the position of the door 400 / engaging element 200 relative to the device 100 may also allow a further reduction in the power usage of the device 100. In particular, if the PCBA 6 receives a signal to release the engaging element 200 (more specifically, to arrange the device such that the engaging element 200 cannot be retained) but it is detected that the engaging element 200 is not proximate the device 100 (or engaged with the device), the device 100 may release the engaging element 200 by retracting the magnet 2 only, without demagnetising the magnet 2. Doing so may save on the need to consume power in demagnetising the magnet 2 when there is no benefit to doing so (because there is no -or minimal -attractive magnetic force between the engaging element 200 and device 100 in such a situation).

Example applications

The above-described system 1000 and device 100 work particularly well for retaining doors 400 as part of fire safety and access control systems. For example, the system 1000 and device 100 may be suitable to fulfil the safety requirements of British standard EN1155 for powered hold-open devices.

It is desirable (and can be a legal requirement) for buildings to have fire doors 400 which close during a fire alarm to prevent the spreading of fire/smoke. Accordingly, fire doors 400 are often fitted with a biasing means 300 (e.g. a door closer) that bias the doors 400 to their closed positions. However, when there is no fire alarm, it is often desirable to keep the door 400 open, e.g. to allow personnel to more easily move across the building.

The device 100 is particularly well suited for use with a fire door 400 because the device 100 can be used to retain the door 400 in the open position, and, upon detecting a fire alarm (via its sensors), automatically release the door 400 to allow it to close, as described above with reference to Figure 9. Once the fire alarm has ended and the further signal received, the magnet 2 is moved to is engaged position and can once again retain the door 400. Further, the device 100 can detect whether the magnet 2 is in the protruded or retracted position (via the position sensor 36 and the movement sensor 40) and transmit a corresponding signal (using the transmitter), e.g. to a building controller, and so the device 100 can provide an assurance that the fire door 400 has been properly released.

It will be appreciated that the device 100 is also well suited to retaining doors 400 and releasing them upon detection of alarms other than a fire alarm -e.g. a carbon monoxide alarm, or a burglar alarm.

It may also be desirable to selectively retain doors 400 in non-emergency situations. For example, in an office, hotel, school, or hospital, it may be useful to retain (e.g. fire or other) doors 400 across the building in an open position during the day, and release the doors 400 so that they close at night. In such a system, a plurality of devices 100 attached to a plurality of doors 400 may be provided, where each device 100 is (at least in part) remotely controlled by a remote server by transmitting control signals to the PCBAs 6 of each device 100. Further, each device 100 can transmit information relating to the current position (i.e. retracted vs. protruded position) of its magnet 2 to the remote server, which can be used e.g. in verifying that all doors 400 are properly closed. The device 100 may also be used in an access control system.

It will be appreciated that in typical use cases such as those described above the device 100 would be 'activated' (i.e. controlled so as to deactivate the magnet 2 to release the door 400) relatively infrequently -for example, once a month or once a week (e.g. when fire systems are tested), or once a day (e.g. to close a door 400 at the end of each day). Thanks to the above-described power saving features of the device (e.g. almost no power consumption in either of the two stable states, and minimised power consumption when changing states), the device 100 may operate for long periods of time (e.g. around 1 year or longer) without the need to replace its batteries. The batteries may be capable of operating for up to around 13,000 'activations' of the device.

Alternative Examples and Embodiments It will be understood that the invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

The present disclosure is described with particular reference to retaining a door 400 which acts as an example of an object retained by the described device 100, and to a fire alarm which acts as an example of a control signal for retaining or releasing the door 400. It will be appreciated that the present disclosure is equally applicable to retaining other objects (e.g. blinds/curtains for a window) and to operating on the basis of other control signals.

It will be appreciated that the locations of the device 100 and engaging element 200 could be swapped -i.e. the device 100 could instead be mounted to the door 400, and the engaging element 200 to the wall 500.

It will be appreciated that the various component parts of the device 100 can be attached to one another in a number of alternative ways -e.g. the base 5 can instead, or in addition, be attached to the lid 1 and/or wall portion 4 using an adhesive, latching mechanism, or bayonet mechanism.

In an alternative example, the device 100 is mains powered and comprises a cable for connecting to a mains power supply.

It will be appreciated that the device 100 can comprise alternative mechanisms for moving the magnet 2 between the protruded and retracted positions. In an alternative example, the magnet 2 does not comprise the aperture 35, and instead the coupler 16 is attached to the plate 3. The coupler 16 is moved axially by the drive outlet 28 (e.g. using corresponding threads on the drive outlet 28 and the coupler 16), thereby axially moving the plate 3 and magnet 2. In a further alternative example, the device 100 instead comprises a pneumatic mechanism (e.g. an axial pneumatic cylinder, or a pneumatic scissors lift mechanism) attached to the base 5 and plate 3, and powered by the motor 8 for protruding and retracting the magnet 2.

It will be appreciated that alternative types of sensors can be used for the position 36 and rotation 40 sensors -e.g. the rotation sensor 40 can comprise a magnetic (e.g. Hall-effect) sensor, and the position sensor 36 can comprise an electromechanical limit switch actuated by the member 38.

In an alternative example, instead of using a coil, the PCBA 6 passes an alternating current through at least part of the magnet 2 to demagnetise it.

In an alternative, the PCBA 6 may be configured to detect the end of fire alarm condition or other alarm condition, and then to actuate the motor 8 such that magnet 2 is moved into the extended position.

The sensors may in an alternative be adapted to differentiate between different types of alarms based on their sounds, and to provide a corresponding input to the PCBA 6.

The PCBA 6 of the device 100 may optionally monitor the power level of the batteries 10. The PCBA 6 can notify an external server of the battery 10 power level, e.g. the PCBA 6 may notify the server if the battery 10 level drops below a pre-defined threshold (e.g. 5%). This can facilitate maintenance of the device 100 by notifying an operator when batteries 10 need replacement. Further, this can allow ensuring that the door 400 is in a safe position in the event of a power outage. For example, if the device 100 retains a fire door 400, and the battery 10 level drops below the pre-defined threshold, the PCBA 6 may release the door 400 (by demagnetising and retracting the magnet 2) to ensure the door 400 is not retained if a fire subsequently breaks out.

In an alternative (though less desirable) example, the magnet 2 is not a permanent electromagnet, but instead the magnet 2 is made from a magnetically "soft" material (e.g. annealed iron) which can be magnetised but tends not to stay magnetised, and/or the device 100 comprises a coil and wire electromagnet (which acts as a magnet when a current is passed through it, but not when the current stops), optionally the coil being wrapped around the "soft" magnet 2. In this alternative example, the magnet 2 is demagnetised in its 'rest' (i.e. no power input) state and requires a power input to be magnetised. Thus, for example, at step 604 of method 600, to demagnetise the magnet 2, current supply (to magnetise the "soft" magnet 2 and/or to the coil and wire electromagnet) is stopped.

In a further alternative example, the magnet 2 is moved/translated non-axially between the protruded and retracted positions.

It will be appreciated that the device 100 and system 1000 described herein may be based on a different attractive force, instead of, or in addition to, a magnetic attractive force between the device 100 and engaging element 200. For example, the device 100 may retain the engaging element 200 via suction, e.g. by creating a vacuum between the device 100 and engaging element 200.

In a yet further alternative example, rather than retaining the door 400 in its open position as in system 1000, the device 100 is arranged such that, when it engages with the engaging element 200, it retains the door 400 in its closed position such that a user cannot open it. In this alternative example, to open the door 400 (which is biased open), the device 100 must be disengaged from the engaging element 200.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims (25)

1. Claims 1 A device for selectively retaining an object, the device comprising: a magnetic element; means for demagnetising the magnetic element; and means for moving the magnetic element between a first position and a second position.
2. 2 A device according to Claim 1, wherein, in the first position, the magnetic element is capable of engaging with the object whereby to retain the object; and, in the second position, the magnetic element is not capable of engaging with the object whereby to retain the object.
3. 3. A device according to any preceding claim, further comprising a controller adapted to control the means for demagnetising and the means for moving.
4. 4 A device according to Claim 3, wherein, upon receipt of a predetermined signal, the controller is configured to control the means for demagnetising to demagnetise the magnetic element; and (preferably simultaneously or immediately subsequently) to control the means for moving the magnetic element to move the magnetic element to the second position.
5. A device according to Claim 4, wherein the predetermined signal is one of: a wireless data signal, a signal associated with a user-actuatable switch, and an alarm, more preferably a fire alarm.
6. 6. A device according to any of Claims 4 or 5, further comprising means for detecting the predetermined signal, preferably wherein the means for detecting comprises a microphone.
7. 7 A device according to any of Claims 4 to 6, wherein, upon receipt of a further predetermined signal, the controller is configured to control the means for moving the magnetic element to move the magnetic element to the first position.
8. 8. A device according to any preceding claim, wherein the means for demagnetising the magnetic element is adapted to demagnetise the magnetic element by passing a current through the magnetic element.
9. 9 A device according to Claim 8, wherein the means for demagnetising the magnetic element is adapted to demagnetise the magnetic element by passing a current through the magnetic element for a predetermined period of time, preferably wherein the predetermined period of time is selected such that the magnetic element moves at least part-way, preferably part-way, from the first position to the second position by the end of the predetermined period of time; more preferably wherein the predetermined period of time is less than 1 second, more preferably less than 500ms, yet more preferably between 100 and 200ms, still more preferably around 150ms.
10. 10.A device according to any preceding claim, wherein the means for moving the magnetic element between a first position and a second position is configured to move the magnetic element between the first and second positions along a straight line, preferably wherein said movement is respectively towards and away from an outer surface of the device.
11. 11 The device of any preceding claim, wherein the means for moving the magnetic element between a first position and a second position comprises a motor configured to move the magnetic element between the first and second positions; preferably wherein the means for moving the magnetic element between a first position and a second position further comprises a screw mechanism configured to receive rotational drive from the motor; more preferably wherein the magnetic element comprises a threaded passageway for receiving the screw mechanism thereby to allow the magnetic element to be driven along an axis of the screw mechanism.
12. 12. The device of any preceding claim, further comprising an outer surface, preferably facing the object, wherein the second position is further away from the outer surface than the first position.
13. 13. The device of any preceding claim, wherein the first and second positions are spaced by between 2 and 10 mm, more preferably 2 and 5 mm, yet more preferably around 3 mm.
14. 14. The device of any preceding claim, wherein the magnetic element and the means for moving the magnetic element are movable relative to an outer surface of the device; preferably wherein the magnetic element and the means for moving the magnetic element are adapted to move away from the outer surface upon application of a force on the device.
15. 15. The device of any preceding claim, wherein the device includes one or more sensors adapted to detect a position of the magnetic element, preferably wherein the device is adapted to transmit data relating to the position of the magnetic element to a server or the controller.
16. 16. The device of any preceding claim, wherein a position of the magnetic element is visible from the exterior of the device, preferably wherein the outer surface comprises an opening showing at least part of the magnetic element.
17. 17. The device of any preceding claim when dependent on Claim 3, wherein the controller is adapted to detect the position of the object relative to the device; preferably wherein said detection includes detecting whether or not the object is engaged with the device.
18. 18. The device of Claim 17, wherein the controller is adapted to detect the position of the object relative to the device by controlling the means for demagnetising to demagnetise the magnetic element for a further predetermined period of time; and monitoring for a change in the inductance of at least part of the magnetic element.
19. 19 The device of Claim 18 when dependent on Claim 9, wherein the further predetermined period of time is less than the predetermined period of time; preferably wherein the further predetermined period of time is selected such that the object, if engaged with the device, remains engaged or is re-engaged by the device at the end of the further predetermined period of time; more preferably wherein the further predetermined period of time is less than 50ms, yet more preferably less than 20ms, still more preferably around 5ms.
20. 20. The device of any preceding claim, wherein the device is battery-powered.
21. 21. The device of any preceding claim, wherein the device is adapted for mounting to a surface, preferably a wall.
22. 22. A device for selectively retaining an object, the device comprising: a magnetic element; means for detecting an occurrence of an alarm, preferably a fire alarm; means for demagnetising the magnetic element, preferably upon detection of an alarm.
23. 23 A method of releasing an object being selectively retained by a magnetic element on a retaining device, the device preferably being the device of any preceding claim; the method comprising: demagnetising the magnetic element; and moving the magnetic element from a first position to a second position, preferably wherein, in the first position, the magnetic element is capable of engaging with the object whereby to retain the object; and, in the second position, the magnetic element is not capable of engaging with the object whereby to retain the object.
24. 24 A system comprising: the device of any of Claims 1 to 22; an object to be retained by the device; preferably wherein the object is adapted to cover an opening, more preferably wherein the object is a door; and preferably, means for mechanically biasing the object away from the device, preferably such that the object moves away from the device when the device selectively disengages from the object.
25. 25. A system comprising: a plurality of devices according to any of Claims 1 to 22; and a server adapted to transmit a signal to each device to release the object retained by each device from being retained by the device.